When you have main-street adoption, like Newegg, Overstock, etc., you'll have a lot of coins being passed from business to exchanges. I think this is why the price is going down steadily this year. Don't get me wrong, it's a good thing people are spending their coins. However, you have to expect businesses to move them to fiat as soon as they get them, which puts more coins on the market at market (or below) rates. We're just not keeping coins between "us" anymore.
Actually after some practice, Bitpay or Coinbase will discover that they earn more by building their reserve to not sell coin when exchange rate is low, that will also calm down the volatility
Bitcoin market capital is still relatively small, it is not difficult for a single entity (for example Bitpay) to maintain a stable exchange rate if they have a large fiat money reserve (The downside risk is known but upside poential is unknown). However, if one bitcoin worth millions of dollars, then no one could manage the volatility alone. Or maybe, by that time, Bitpay already accumulated huge amount of fiat/bitcoin reserve, thus still in control